1. Field of the Invention
This invention relates to the field of integrated inductors, and particularly to monolithic high-Q transformer-type active inductors.
2. Description of the Related Art
Many analog circuit designs, such as fully integrated RF transceivers, require high performance reactive componentsxe2x80x94particularly inductors. It is often desirable for inductors in RF transceivers to be very low loss, with a resulting high quality factor (Q). For instance, high-Q tank circuits in voltage-controlled oscillators (VCOs) reduce phase noise. High-Q inductors in low noise amplifiers (LNAs) and power amplifiers (PAs) reduce noise, and improve gain, efficiency and input/output matching.
Bandpass filters and resonators crucial to many multi-function communication systems are commonly made of off-chip inductors and capacitors based on SAW or ceramic technologies, which are expensive, bulky, complex in assembly and problematic in reliability. In the past, the integration of these filters or resonators with CMOS components was as regarded impossible, due to the lack of integratable high-Q inductors to meet system needs in insertion loss, linearity and switching speed.
One approach to these problems is discussed in U.S. Pat. No. 5,994,985 to Pehlke et al. There, two coils are driven with respective currents to provide an active inductor. The design requires the use of a variable gain amplifier, a variable phase shifter, and a directional coupler. However, the circuit described in Pehlke et al. has a number of deficiencies. The directional coupler, for example, is too large to be practically integrated. With the coupler off-chip, a hybrid circuit implementation is required, which tends to be more costly and unreliable than a monolithic design; a hybrid implementation also suffers from higher power consumption and noise, has limited linearity, and requires a larger area.
Furthermore, the variable phase shifter itself often requires high-Q inductors to facilitate phase tuning. Also, the cited patent describes only a one-terminal (grounded) active inductor, which is inadequate in many circuit designs.
A monolithic active inductor circuit is presented which overcomes the problems noted above. Each inductor circuit includes a primary and a secondary coil and a drive circuit, all of which are monolithically integrated on a common substrate to provide a high-Q value; the resulting inductor can in turn be used to provide high performance on-chip resonators and filters.
Each monolithic active inductor circuit comprises an input terminal which receives an RF input signal, a primary coil which carries a first current that includes an AC component which varies with the RF input signal, and a secondary coil which carries a second current that also includes an AC component which varies with the input signal. The primary and secondary coils are located in close proximity to each other such that a magnetic field induced by a current in the secondary coil is coupled to the primary coil.
The active inductor circuit also includes an on-chip current source which provides the second current in the secondary coil. The inductor circuit is arranged such that there is a fixed phase difference of approximately 90xc2x0 between the AC components of the first and second currents and such that the magnetic field induced by the second current compensates for energy that would otherwise be dissipated by the primary coil. When the second current is properly selected, the inductor circuit""s input impedance is made purely imaginary, and the circuit emulates an ideal inductor at a particular frequency.
The on-chip current source preferably comprises a single transistor or a current mirror circuit, preferably CMOS, and the fixed phase difference is preferably achieved with an on-chip capacitor. When so arranged, the inductor coils and current source are easily fabricated on a common substrate, with the current source circuit preferably using CMOS components.
The present active inductor circuit can be combined with on-chip capacitors to form fully integrated high performance resonators and filters, using well-known and cost-effective fabrication techniques.